Hi Kevin, I had to look on line to find the answer, here's what I found.

'The scientists from Oxford University - where ivy adds colour and character to many of the college buildings - found that rather than damaging walls, the plant positively protects them by acting as a 'thermal shield', insulating brickwork from the extremes of temperature and moisture that often cause cracks. It can also protect against pollution damage. However, the plant has not completely been given the all clear. Although it was a boon for buildings that were intact, the research showed that where walls are already damaged, ivy finds its way into cracks and holes. The plant is unlikely to worsen damage on solid brickwork though, with the effect mainly cosmetic. The research team found that ivy can be safely taken off leaving walls intact.'